1. Field of the Invention
The present invention pertains to semiconductor switching devices and in particular to semiconductor thyristor devices having branched base structures.
2. Description of the Prior Art
It is known in the art to construct thyristor devices with low resistivity branches in the base zone. Such branches produce a reduction in the transverse resistance of the base which has several favorable effects. One favorable effect is a reduction in turn-off time. A second favorable effect is an improved capability for operation in the gate turn-off mode.
Briefly, gate turn-off is achieved by impressing a voltage on the base of a thyristor device operating in its low impedance state, which voltage has a polarity with respect to the emitter opposite to that normally used to turn on the device.
There are two families of structures in the prior art which provide such low resistivity branches.
First, there are thyristors wherein the base zone has branches which pass through the emitter zone emerging at the semiconductor surface. Such devices require geometrically complex electrodes.
Second, in an effort to eliminate complex electrodes, devices have been proposed wherein the base zone comprises low resistivity branches running underneath the emitter zone, the low resistivity being produced by an epitaxial process which results in nonuniform doping concentration relative to the same level of the base zone separating the branches.
The present invention overcomes the disadvantages of such prior art devices and provides a fast-switching thyristor device suitable for operation in the gate turn-off mode which comprises a novel emitter-base structure having low resistivity branches in the base zone. The present invention provides a two-level emitter zone, which renders unnecessary the above-described, nonuniform, epitaxially- produced doping concentration.